UV-Visible and near infrared magnetic circular dichroism (MCD) spectroscopy and rapid scan stopped flow spectroscopy will be used to study heme iron enzymes and related systems. MCD, along with resonance Raman spectroscopy, will be used to study the H93G cavity mutant of myoglobin ligated by amine, carboxylate or phenolate ligands. Additional MCD studies will include heme protein maquettes and the transcription factor NPAS2. Studies of the cavity mutant and the heme protein maquettes will expand our data base of MCD spectra for structurally defined heme systems, which we will then apply to structurally uncharacterized heme iron systems such as NPAS2. Identification of axial ligands can give insight into heme enzyme mechanisms. The NPAS2 protein binds two heme PAS domains that appear to regulate the activity of the transcription factor. Stopped flow studies will explore the redox role of the tetrahydrobiopterin cofactor in the catalysis of nitric oxide synthase (NOS) by conducting single and double mixing experiments with six different pterins and three different substrates. Studying NOS catalysis will allow for a better understanding of the biosynthesis and regulation of nitric oxide gas (NO), the signaling product of NOS catalysis. [unreadable] [unreadable]